How do clouds form?
Assume you have a volume of air to which you can add water vapor by opening a valve of,
as an example, a high pressure boiler. If you
observe the volume of air while you continuously
add water vapor from the boiler, you will notice that at some
point tiny droplets begin to appear and the walls of the vessel
containing the air moisten. The point at which this condensation
occurs within the moistened air is when the air
is saturated with water vapor, which means it cannot hold more
water in the form of vapor. At this point,
called the saturation point, the volume of air
must release some of the water vapor it
contains, doing so in the form of the tiny droplets or as moisture on walls.
Let's call the concentration of water vapor in
the volume of air at this point rsat.
If you recreate this experiment, but this
time at different temperatures, you will notice that the ability of
the volume of air to hold water vapor increases strongly with
temperature. In other words, the warmer air can hold much more
water vapor than the cold air. We can
represent this as rsat(high temp.)>
rsat (low temp).
When we reach the saturation point in our experiment, the so called relative humidity** - RH- is 100%.
Before you reach saturation, RH is less than 100%. When you reach 100%, droplets are formed in our
experiment. Similarly, in the atmosphere a cloud would form.
Source: ESPERE - eu
THE WATER VAPOR EXPERIMENT
The animation presents the experiment schematically. We let some water
vapor from the boiler (green) into a reaction chamber (black) filled with dry air. Let's assume we could count the billions of molecules (blue dots) and we observe these few ones as representatives of the very large number.
a) The maximum concentration leading to
saturation is assumed to be 50 molecules (max = rsat).
After the addition of the first 25 particles the relative humidity is 50%. But we add some more. When the number exceeds 50 (RH > 100%) droplets are formed on the wall of the chamber.
b) Now we heat the chamber (shown in red).
The ability of the air to take up water molecules increases to 90 (max = rsat).
The molecules already condensed on the wall evaporate again.
c) Instead of heating the chamber even more, we will cool it down now until the temperature is significantly below the value at the beginning (chamber in light blue).
Now the cold air can only keep 30 molecules without condensation.
The air, containing 95 molecules, is now
oversaturated. The water condenses and forms a little lake at the bottom of the chamber.
It has rained in a way.
|(**) To quantify the amount of water vapor
in an air mass we can introduce the
concept of relative
humidity. The relative humidity is
the ratio (expressed in %) between the ambient water vapor "concentration"
r and the saturation water vapor "concentration"
If RH=100%, then there is equilibrium
between condensation and evaporation.
If RH<100%, then there is
a dearth of vapor and evaporation is dominant.
If RH>100%, then there is
an excess of vapor and condensation is dominant.
As is clear from the name, relative humidity is a relative value.
If you cool down air containing
a concentration r
of water vapor
by 10°C, the relative humidity
increases, because the saturation concentration
decreases. Thus RH depends on
temperature and not the actual
concentration of the water
Now you've seen that you can form cloud
droplets by opening a valve and adding water vapor to
a volume of air. But a clever reader may also
have noticed that droplets can be formed by cooling the air while your valve remains closed.
In this case, you simply lower the air's holding capacity for vapor until droplets are formed.
Nature has examples for us of both types of
- adding water vapor to the air mass: your breath in cold air, aircraft condensation trails
- cooling the air mass: fog near cold ground in autumn and winter
In general, nature uses the second process to
form cloud, cooling an air mass by lifting the air and bringing it into a cooler
environment. There are several ways to lift the air:
Ground warming (e.g. by sun rays in the morning) makes the
air adjacent to the ground warmer, and thus less
dense and more buoyant, than the air directly
above the warmed air. Thus the air
rises. Although the air mass cools down as
it rises through the atmosphere, due to its
expansion, it is still warmer and lighter than the air
around it up to a certain point. Until it
reaches this point, the process of cooling,
expansion, and rising continues.
[Orography is the description of the relief of a landscape. Orography
describes how the shape of the landscape is formed by mountains, plains, hills, forests, etc.]
An important process is the lifting of the air on the windward side of a mountain:
source: ESPERE - E.U.
Animation: This phenomenon often ends up in the formation of thunderstorms if the moisture climbs up the slope of the mountains during a warm summer day, and
then cools down when it reaches higher altitudes.
The obstacles that make approaching air rise
do not have to be land surface features, i.e. another air mass of different
temperature could also make approaching air
rise, due to differing densities between the air
masses. The surface separating the two air masses is called a "front".
Front configurations are found, as one example,
as a feature in low pressure systems. There warm air is lifted at the front
by the previous cold air mass (warm front) or the cold air pushes warm air in higher altitudes (cold front). Along these fronts clouds
typically form, and frequently can give rise to
source: WW2010 of University of Illinois
|Cloud Condensation Nuclei
Aerosols particles act as Cloud Condensation Nuclei.
In the last part we have discussed which conditions are needed to cause condensation.
In order to have condensation and the growth of the droplets by diffusion of vapor,
though, we must somehow form the droplets!
In theory, at the beginning of droplet formation, one or two water molecules
should collide and stay together. Others should join them,
and eventually should form a tiny droplet of about 1 µm
in size. This process is called spontaneous
nucleation. This should happen and
clouds should form, correct? Well, not
really. Consider the following
Observe an air parcel under a very strong microscope. We carry out the steam chamber experiment above in the real air and assume again we would be able to count the molecules. We choose 60 molecules in our air parcel.
In the air column, a figure near
the maximum number of molecules that can be taken up without condensation is indicated. The higher the altitude of the air parcel, the colder the air and the smaller the value of
rsat. And we see ...
that we don't see anything.
What is going on? No condensation?
In fact this observation is in agreement with
reality. After the formation of a small molecule cluster, other molecules
should join it in order to form a tiny droplet of about 1 µm.
However, in order to keep together a handful of water molecules, an enormous
atmospheric pressure would be necessary. As this pressure needs to come from the environmental gas, this would mean a vapor concentration equivalent to a relative humidity of
much more than 100%, about 200-300%. This is why, instead of 60 molecules, about 150 to 200 would be necessary at the highest point of our air column in order to
incur spontaneous nucleation. Such values,though, have never been observed
in nature (although we do observe clouds with slight
supersaturation of the air, perhaps a few
percent over 100%). In any case, is there a bug in the theory, or
are we missing a necessary process?
As you might have guessed, we are missing
a process that makes the droplet formation
possible. We need a surface upon which
the droplets can accumulate, e.g. the walls
and floor of the chamber in the steam chamber
experiment in the first paragraph. The
surfaces in the atmosphere that perform this
function are called aerosols, or small solid particles such as dust, sea spray,
sulfates, etc. As these particles offer a
relatively large surface, condensation onto the particles is possible with relative humidities
in the atmosphere only slightly above 100%.
In the air there are a large amount of these suspended particles,
on the order of micron and submicron size, some of which have an affinity for water and serve as centers for condensation.
These particles are called cloud condensation nuclei for the
water droplets (ice forming nuclei for the ice
crystals). This microphysics process is called heterogeneous nucleation.
In a new light, then, let's observe again the same air parcel,
but now including the aerosols (brown) in our picture. They are providing the surface
upon which the water molecules condense. They form tiny droplets when saturation is reached.
Thank goodness! Theory and observation are
once again in agreement!
|The cloud - a chemical reactor
A great variety of gases, solids, and liquids
are in the air, and clouds can account for a
very high percentage of those particles in a
given region. There are three primary
methods by which a cloud takes up materials in
the air. First, a large number of
particles are incorporated in the condensed phase of the cloud, as cloud condensation nuclei or ice forming nuclei.
Second, aerosol particles can be removed from the atmosphere by collision with drops or crystals.
Finally, since many gases are soluble in water,
they will be fixed by the condensed phase of the cloud.
It is via these three methods that the cloud can "scavenge"
pollutants from the air. Thus, if the cloud rains, the air will be
temporarily cleaned of many of these chemicals.
The drops remaining in the air will eventually evaporate and release their
chemical components once again, and the cycle
Clouds are formed when the relative humidity in the air is higher than 100%.
A slight supersaturation is sufficient,
since the condensation takes place on the surface of aerosol particles
ubiquitous in the air. In most cases a supersaturation is caused by the lifting of warm air masses which cool down. This can be caused by convection, orography or fronts.
Text: Marie Monier - Université Blaise Pascal de Clermont Ferrand / France
Supported by: Klaus Gierens from DLR, who wrote a big part of the humidity paragraph
Reviewing and corrections: Prof. Andrea Flossmann - Clermont Ferrand;
Stephen Gawtry - University of Virginia
Animations and their description: Elmar Uherek - MPI Mainz